The present invention generally relates to chemical mechanical polishing (CMP). In particular, embodiments of the invention relate to detection of endpoints in CMP processes.
Polishing of semiconductor wafers by CMP during fabrication of integrated circuits is an accepted practice in the semiconductor industry. Typically, a wafer to be polished is secured to a head, and then placed into contact with a polishing pad in combination with a slurry.
In certain CMP processes, it is desirable to remove one or more layers of material on the wafer, and then to stop the polishing process on an underlying layer of a different material. For example, in a damascene process copper may be formed within a silicon oxide trench featuring a tantalum liner. A CMP step to remove copper and tantalum outside of the trench may end upon encountering oxide on surfaces adjacent to the trench.
Conventionally, endpoint of CMP processes is identified as a function of time during process development. During actual processing, the CMP step is timed, and endpoint determined indirectly, in order to produce desired polishing results.
However, polishing rates can vary depending upon the actual parameters of the CMP step, such as rotation rate, loading force, and the precise composition and identity of the slurry. Accordingly, conventional timed polishing techniques may result in removal of excessive amounts of material, or may result in too little material being removed. Either result is undesirable from a process repeatability standpoint.
Other conventional techniques for determining CMP endpoint include monitoring frictional coefficient between the polishing pad and the wafer, with a change in frictional coefficient indicating a transition in polishing between layers. While effective, this approach to CMP endpoint detection is dependent upon the precise composition and identity of the slurry used in the polishing step. Use of a different slurry, or even use of the same slurry at slightly different mixtures, can have a significant effect upon the frictional coefficient.
Therefore, structures and methods that accomplish accurate and reliable detection of the endpoint of chemical-mechanical polishing processes are desirable.
Embodiments of the present invention provide methods and apparatuses for detecting endpoint in a CMP process. Specifically, acoustical emission information produced by sliding contact between the polishing pad and different material layers on the wafer is monitored using an acoustic information sensor. This acoustic information is resolved into a frequency spectrum utilizing such techniques as fast Fourier transformation. Characteristic changes in the acoustic frequency spectrum reveal any transition in polishing between different material layers. The CMP endpoint indicated by changes in the acoustic frequency spectrum is validated by correlation with other sensed properties, including but not limited to changes in the amplitude of acoustic energy over time, and a change in the measured frictional coefficient between wafer and pad. CMP endpoint can also be validated by comparison with characteristic AE frequency spectra obtained at endpoints of prior CMP operational runs.
An embodiment of a method for detecting transition between polishing of material layers during a chemical mechanical polishing process comprises sensing acoustical energy generated by contact between a chemical mechanical polishing pad and a semiconductor wafer. The sensed acoustical energy is converted into an electrical signal, and a low frequency component of the electrical signal is filtered. The filtered electrical signal is resolved into a frequency spectrum. A difference between the frequency spectrum and a previously obtained acoustic emission frequency spectrum is identified. The difference is correlated with a transition in polishing between layers of material on the semiconductor wafer, and the transition is validated with reference to a change in a separate indicia from the CMP process.
An embodiment of a method for detecting endpoint of a CMP process comprises sensing a first acoustical energy generated by contact between a chemical mechanical polishing pad and a first semiconductor wafer at a transition between a first material and a second material during a first CMP operational run. The first acoustical energy is resolved into a characteristic transition frequency spectrum. The characteristic transition frequency spectrum is stored in a memory. A second acoustical energy generated by contact between the chemical mechanical polishing pad and a second semiconductor wafer during a second CMP operational run is sensed. The second acoustical energy is resolved into a sensed transition frequency spectrum. The characteristic transition frequency spectrum is compared with the sensed transition frequency spectrum to identify a CMP endpoint during the second operational run. The CMP endpoint is validated with reference to a change in a separate indicia from the second CMP operational run.
An embodiment of an apparatus for detecting an endpoint of a chemical mechanical polishing process in accordance with the present invention comprises an acoustic emission sensor positioned proximate to a chemical mechanical polishing pad. The sensor includes a transducer configured to convert acoustical energy generated by contact between the pad and a semiconductor wafer into an electrical signal. A second sensor is configured to detect non-acoustic information from the process. A memory is configured to store a previously obtained acoustic emission frequency spectrum. A low frequency filter is in electrical communication with the transducer. A computer is in electrical communication with the filter, the second sensor, and the memory, the computer configured to resolve the electrical signal into a frequency spectrum and to identify differences between the frequency spectrum and the previously obtained acoustic emission frequency spectrum in order to determine a transition between polishing of different materials, the transition corresponding to an endpoint.
These and other embodiments of the present invention, as well as its features and some potential advantages are described in more detail in conjunction with the text below and attached figures.